Manufacturing a photodetector device often exposes the device to processes, materials, and techniques that result in limiting their operating range or hindering their performance. For example, a photodetector may comprise a combination of p-type and n-type semiconductor materials, with each material offering different electrical properties that work in combination to provide a functional device. The steps by which these devices are fabricated, however, can alter these materials in ways that affect the device's operation. An infrared detector is a notable example of how the steps in the fabrication process can cause these effects.
Infrared detectors can be fabricated using an infrared absorbing semiconductor material, referred to as “absorption material”, that offers an electrical path through which, when the device is operating, bulk electrical current will flow. The bulk electrical current is typically used as an environmental indicator of when the device is exposed to an environment having infrared energy. When fabricating these devices, though, the dry and wet chemical etching processes used can alter the semiconductor absorption material at the surface. More specifically, these processes can create a conductive surface layer that bypasses the path through which bulk electrical current should flow. In such instances, the device will be characterized by the surface leakage current flowing through the conductive surface layer rather than the bulk current that should be flowing through the semiconductor absorption material. Consequently, surface leakage current can become a significant concern because it hinders the device's reliable operation.
The challenges posed by surface leakage current can be particularly prevalent in photodetector devices using p-type absorption materials, where the etching process results in “pinning” at the surface. The term “pinning” is used to describe the bending of conduction and valence bands similar to what happens in the depletion region of a “pin” diode. This pinning, in turn, leads to an accumulation of electrons near the surface such that the p-type material becomes an n-type material. This change in doping type creates what is known as a “surface inversion” where the concentration of dopants is higher at the surface where the n-type material is prevalent and lessens as one moves deeper into the absorption material until the p-type material is prevalent. This surface inversion defines the surface layer. What is needed in such devices is an ability to convert the n-type material at the surface layer back to a p-type material, and vice versa when the absorption material is an n-type material.
A variety of techniques known in the art have attempted to compensate for these effects, some of which have employed a type of passivation layer as a protective film. Such passivation layers are typically not doped. Indeed, as indicated in U.S. Pat. No. 9,276,162 to Yasuoka et al. (hereinafter “Yasuoka”), deposited passivation layers on the sidewall of a photodetector are comprised of, for instance, undoped indium phosphide. In addition to being undoped, such a passivation layer has the disadvantage of having to be epitaxially regrown on the photodetector, requiring a second deposition process and thereby resulting in a more complicated and less flexible fabrication process than if alternative methods, including various deposition techniques, were available.
Additionally, Yasuoka teaches a process of diffusing zinc from a top of a mesa structure defined by an insulator mask into the side areas of a multiplication region, as opposed to into a surface layer directly. When employed, such techniques tend to reduce the thickness of the multiplication region and increase the breakdown field at the edges of the mesa structure. These effects further tend to increase the residual electric current flowing through the photodiode when no incident electromagnetic radiation is present (i.e., dark current). Such techniques, however, do not address the pinning phenomenon described above because they are unable to diffuse a dopant across a junction of the depth necessary to limit surface leakage current. Moreover, if mesa fabrication is performed after dopant diffusion, then chemical etching can still be required as part of the fabrication process, and when employed, this etching will tend to convert the side areas back to an n-type material as described above.
Accordingly, there is a need in the art to compensate for the effects of the fabrication process on the reliability and operation of photodetectors. More specifically, there is a need to passivate a surface layer in a photodetector device to reduce surface leakage current and/or dark current that would hinder device performance, particularly in those devices that utilize p-type semiconductors as an absorption material. This passivation could be done as part of a post-fabrication process or in conjunction with other fabrication steps.
This background information is provided to reveal information believed by the applicant to be of possible relevance to the present invention. No admission is necessarily intended, nor should be construed, that any of the preceding information constitutes prior art against the present invention.